Make CLI bootstrap Poetry-aware

[NiklasRosenstein]

Summary

• create pydoc-markdown.yml when bootstrapping beside a Poetry-only pyproject.toml
• refuse to shadow an existing [tool.pydoc-markdown] configuration
• warn only when quick CLI options actually ignore a usable local configuration
• preserve YAML-over-TOML precedence and create bootstrap files exclusively to avoid symlink/race overwrites
• document the workflow and add focused CLI regression coverage

Closes #197.

Validation

• full pytest suite: 106 passed
• canonical mypy check passes
• Ruff lint and format checks pass
• Python 3.10 bootstrap smoke test passes
• independent review findings fixed; final re-review found no issues
• diff check passes

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